Thank you all for explainations and links.however its not easy to understand completly.for example, we know there wlll be 21 m bitcoin.if we are talking about btc/usdt and if there will be more tether than 21m what will happen? This is literally inflation.plus there are many stable coins not only tether.
You are incorrect. Tether cannot harm Bitcoin in anyway, no matter what they do. The same thing was brought by certain someone in another thread, but using fiat instead of an inflationary altcoin.
Tether can "print" all they want, but they would be unable to buy bitcoin in the same fashion as they print, because every extra Tether created and used to buy those bitcoins will cause the price (in USDT) to rise. There is no way around it, they just can't, no one can.
Also, do not fall for Tether lies. They are using fractional reserve, try to research a little on that, its what banks do. Hint: Its the sole reason banks can go bankrupt. A bank with full reserve, can never go bankrupt.
This should give you the hint: Tether can crash, hard. Bitcoin won't be affected by that.